Compound semiconductor, method for manufacturing same, and nitride semiconductor

ABSTRACT

A compound semiconductor has a high electron concentration of 5×1019 cm−3 or higher, exhibits an electron mobility of 46 cm2/V·s or higher, and exhibits a low electric resistance, and thus is usable to produce a high performance semiconductor device. The present invention provides a group 13 nitride semiconductor of n-type conductivity that may be formed as a film on a substrate having a large area size at a temperature of room temperature to 700° C.

TECHNICAL FIELD

The present invention relates to a compound semiconductor and a methodfor manufacturing the same.

BACKGROUND ART

Devices using group 13 nitride semiconductors such as GaN and InN arenow put into practical use in a wide range of products. Conventionally,an MOCVD method and an MBE method have been used for crystal growth ofsuch a group 13 nitride semiconductor. However, the MOCVD methodrequires a process temperature exceeding 1000° C. The MBE method allowsa compound semiconductor film to be formed at a low temperature, but isnot suitable to mass manufacturing because there is a limit on the areasize of the film that may be formed and the manufacturing cost is high.

With the MBE method, if donors are incorporated at a high concentration,absorption by the high concentration donor level generated in theforbidden band in the vicinity of the conduction band of the crystalstructure occurs. For this reason, the MBE method has a problem that thetransparency of the manufactured compound semiconductor film isdecreased (Non-patent Document 1). For these reasons, the MOCVD methodis now used to manufacture a compound semiconductor, mainly, tomanufacture a nitride semiconductor fora practical use.

Currently, next-generation electronic devices having both a highwithstand voltage and a low on-resistance are desired. In order torealize such an electronic device, it is desired to realize a two-,three-, or four-component compound semiconductor, more specifically, acompound semiconductor device using a group 13 nitride semiconductor.This requires further improvement in the quality of the crystal of sucha compound semiconductor and improvement in the refinement of the dopingtechnology. Especially for a vertical power device to be formed on a GaNsubstrate, it is urged to decrease the carbon concentration of an n-typedrift layer and to improve the electron mobility. There are thefollowing documents describing the prior art.

Patent Document 1 discloses a semiconductor device including a bufferlayer formed of a metal nitride and a semiconductor layer, which areprovided on a copper substrate.

Patent Document 2 discloses examples of a semiconductor substrateincluding a graphite plate having a thickness of 10 to 100 μm,containing a sintered polymer and having a heat resistance andflexibility, a buffer layer formed of HfN on the graphite plate, and asemiconductor layer formed of GaN on the buffer layer.

Patent Document 3 discloses a method for manufacturing a group-III-Vcompound semiconductor by epitaxial growth on a ZnO substrate.

Non-patent Document 1 discloses research results on formation of ap-type GaN semiconductor layer. Non-patent Document 2 discloses researchresults on the contact resistance of a p-type GaN semiconductor layer.Non-patent Document 3 discloses research results on a low concentrationdoping technology into a nitride semiconductor. Non-patent Document 4discloses research results on a transport model of electrons in a highelectric field. Non-patent Document 5 discloses research results on amodel of carrier mobility of GaN. Non-patent Document 6 disclosesresearch results on evaluation of the contact resistance against ap-type GaN formed by a PSD method. Non-patent Document 7 disclosesexamples of experiments of producing an LED on glass. Non-patentDocument 8 discloses research results on a nitride single crystal grownby the PSD method.

CITATION LIST Patent Literature

Patent Document 1: Japanese Laid-Open Patent Publication No. 2008-243873

Patent Document 2: WO2011/021248A1

Patent Document 3: Japanese Laid-Open Patent Publication No. 2010-56435

Patent Document 4: Japanese Laid-Open Patent Publication No. 2016-115931

Patent Document 5: United States Patent Application Publication2016/0172473

Non-Patent Literature

Non-patent Document 1: G.T. Zhao et al., Jpn. J. Appl. Phys. 38, L933(1999;

Non-patent Document 2: Arakawa et al., The Japan Society of AppliedPhysics, 63rd Spring Meeting, 20p-H121-8

Non-patent Document 3: E. Nakamura et al., Appl. Phys. Lett. 104, 051121(2014)

Non-patent Document 4: D. M. Caughey et al, Proc. IEEE 55, 2192 (1967)

Non-patent Document 5: T. T. Mnatsakanov et al, Solid-State Electron,47, 111 (2003)

Non-patent Document 6: Proceedings, The Japan Society of AppliedPhysics, 62nd Spring Meeting

Non-patent Document 7: Nikkei Electronics, NE report, pp.14-15, Jul. 7,2014

Non-patent Document 8: Fujioka “Flexible Device” project researchabstracts pp.89-94 (published on Mar. 4, 2008)

Non-patent Document 9: A, Suzuki et al., “Extremely low on-resistanceEnhancement-mode GaN-based HFET using Ge-doped regrowth technique”(IEDM14, pp.275-278 (2014))

SUMMARY OF INVENTION Technical Problem

With the conventional technology, in the case where it is attempted torealize crystal growth of a group 13 nitride semiconductor by the MOCVDmethod, carbon and hydrogen contained in the material gas areincorporated into the film. This causes a problem that it is difficultto form a high quality film having a low concentration of impuritiessuch as carbon, hydrogen and the like.

In addition, in the case where it is attempted to realize crystal growthof a group 13 nitride semiconductor by the MOCVD method, it is generallydifficult that a film having a donor concentration of 5×10¹⁹ cm⁻³ orhigher exhibits an electron mobility of about 46 cm²/V·s or higher dueto thermodynamic restrictions. The MOCVD method is based on a chemicalreaction. Therefore, it is in fact impossible to realize crystal growthat a low temperature, and carbon and hydrogen contained in the materialgas are easily incorporated into the manufactured film.

As a crystal growth method of a nitride semiconductor replacing theMOCVD method, a pulse sputter deposition (PSD) method is now proposed.It has been demonstrated that a p-type GaN thin film having a lowconcentration of residual hydrogen and exhibiting a high hole mobilityis obtained by the PSD method (Non-patent Document 2).

Currently, the MOCVD method is used to manufacture an electronic deviceand a light emitting device on a nitride semiconductor substrate forpractical use. However, it is difficult to produce, by the MOCVD method,an n-type layer having a high donor concentration which are important todecrease the resistance of these devices. Therefore, there are very fewreports on the characteristics of such an n-type layer.

As can be seen, it is desired to develop a group 13 nitridesemiconductor of n-type conductivity that exhibits a high electronmobility even in a region of a high donor concentration. In such asituation, it is required to realize a semiconductor material exhibitingas high an electron mobility as possible in order to achieve thepurposes of improving the performance, saving the energy, and improvingthe efficiency of electronic devices and light emitting devices.

The present invention, made in light of such a problem, has an object ofeasily manufacturing and providing a two-, three- or four-componentcompound semiconductor, more specifically, a group 13 nitridesemiconductor film, of n-type conductivity that exhibits a high electronmobility even in a region of a high donor concentration.

Solution to Problem

In order to solve the above-described problem, embodiment 1 of the firstinvention provides a two-, three-, or four-component compoundsemiconductor containing nitrogen and one element selected from thegroup consisting of B, Al, Ga and In, which are group 13 elements,wherein the compound semiconductor contains oxygen as an impurity at1×10¹⁷ cm⁻³ or higher, the compound semiconductor has an electronconcentration of 5×10¹⁹ cm⁻³ or higher and has n-type conductivity, andthe compound semiconductor exhibits an electron mobility of 46 cm²/V·sor higher.

Embodiment 2 of the first invention provides the compound semiconductoraccording to embodiment 1, in which the compound semiconductor containsGa and N as main components.

Embodiment 3 provides the compound semiconductor according to embodiment2, in which the compound semiconductor has an absorption coefficient of2000 cm⁻¹ or lower to light having a wavelength region of 405 nm.

Embodiment 4 provides the compound semiconductor according to embodiment2, in which the compound semiconductor has an absorption coefficient of1000 cm⁻¹ or lower to light having a wavelength region of 450 nm.

Embodiment 5 provides the compound semiconductor according to any one ofembodiments 1 to 4, in which the compound semiconductor has an RMS valueof 5.0 nm or less obtained by a surface roughness measurement performedby an AFM.

Embodiment 6 provides the compound semiconductor according to any one ofembodiments 1 to 5, in which the compound semiconductor has a contactresistance of 1×10⁻⁴ Ω·cm² or lower against an n-type ohmic electrodemetal.

Embodiment 7 provides the compound semiconductor according to any one ofembodiments 1 to 6, in which the compound semiconductor contains Ga asthe group 13 element and further contains Al and/or In as the group 13element.

Embodiment 8 provides the compound semiconductor according to any one ofembodiments 1 to 7, in which the compound semiconductor contains Si as adonor.

Embodiment 9 provides the compound semiconductor according to any one ofembodiments 1 to 7, in which the compound semiconductor contains Ge as adonor.

Embodiment 10 provides a contact structure, comprising a conductiveportion and an electrode connected with each other, the conductiveportion being formed using the compound semiconductor according to anyone of embodiments 1 to 9.

Embodiment 11 provides a semiconductor device, comprising the contactstructure according to embodiment 10.

Embodiment 12 provides a transparent electrode formed using the compoundsemiconductor according to any one of embodiments 1 to 9.

Embodiment 13 provides a method for manufacturing a compoundsemiconductor, the method comprising forming a film using the compoundsemiconductor according to any one of embodiments 1 to 9 by a pulsesputtering method in a process atmosphere containing oxygen.

Embodiment 14 provides the method for manufacturing a compoundsemiconductor according to embodiment 13, in which the film is formed ata substrate temperature of 700° C. or lower.

According to the first present invention, a two-component nitride refersto a compound of one element among B, Al, Ga and In, and nitrogen.Namely, the two-component nitride is a two-component mixed crystal of BN(boron nitride), AIN (aluminum nitride), GaN (gallium nitride) or InN(indium nitride).

A three-component nitride refers to a compound obtained as a result ofany one of the two-component group 13 elements mentioned above beingpartially replaced with another group 13 element. The three-componentnitride is, for example, a three-component mixed crystal of InGaN(indium gallium nitride), AlGaN (aluminum gallium nitride), or AlInN(aluminum indium nitride). Regarding the three-component compound, it isknown that the composition ratio thereof may be adjusted to adjust thebandgap within the range of the characteristics of the two-componentcompound.

Even a compound containing a trace amount of another group 13 element inaddition to the group 13 acting as a main component of the compoundsemiconductor may also be encompassed in the scope of theabove-described invention. Combinations of elements are arbitrary aslong as the effects of the present invention are not impaired.

The second invention is directed to a nitride semiconductor havingn-type conductivity and containing nitrogen and at least one group 13element selected from the group consisting of B, Al, Ga and In, in whichthe nitride semiconductor has an electron concentration of 1×10²⁰ cm⁻³or higher and exhibits a specific resistance of 0.3×10⁻³ Ω·cm or lower.

Preferably, the electron concentration is 2×10²⁰ cm⁻³ or higher.

Preferably, the nitride semiconductor has a contact resistance of 1−10⁻⁴Ω·cm² or lower against an n-type ohmic electrode metal.

In an embodiment, the nitride semiconductor contains oxygen as animpurity at 1×10¹⁷ cm⁻³ or higher.

Preferably, the nitride semiconductor has an absorption coefficient of2000 cm⁻¹ or lower to light having a wavelength region of 405 nm.

Preferably, the nitride semiconductor has an absorption coefficient of1000 cm⁻¹ or lower to light having a wavelength region of 450 nm.

Preferably, the nitride semiconductor has an RMS value of 5.0 nm or lessobtained by a surface roughness measurement performed by an AFM.

In an embodiment, the at least one group 13 element is Ga.

In an embodiment, the nitride semiconductor contains either one of, orboth of, Si and Ge as donor impurities.

The lower limit of the specific resistance is, for example, 0.2×10⁻³Ω·cm, 0.15×10⁻³ Ω·cm, or 0.1×10⁻³Ω·cm.

The relationship between the electron concentration of the specificresistance of the nitride semiconductor fulfills a numerical rangeenclosed by four points at which (a) the electron concentration is1×10²⁰ cm⁻³ and the specific resistance is 0.3×10⁻³ Ω·cm, (b) theelectron concentration is 3×10²⁰ cm⁻³ and the specific resistance is0.3×10⁻³ Ω·cm, (c) the electron concentration is 4×10²⁰ cm⁻³ and thespecific resistance is 0.15×10⁻³ , and (d) the electron concentration is9×10²⁰ cm⁻³ and the specific resistance is 0.15×10⁻³ Ω·cm.

The above-described invention is applicable to a contact structure,comprising the nitride semiconductor for a conductive portion. Theabove-described invention is also applicable to a contact structure,comprising the nitride semiconductor for an electrode. Such a contactstructure is usable in a semiconductor device.

ADVANTAGEOUS EFFECTS OF INVENTION

The nitride compound semiconductor according to the present inventionexhibits a high electron mobility of 46 cm²/V·s or higher even in aregion of a high electron concentration of 5×10¹⁹ cm⁻³ or higher. Theelectron mobility is preferably 50 cm²/V·s or higher, and morepreferably 60 cm²/V·s or higher.

However, the electron mobility of 50 cm²/V·s or higher may not be neededdepending on the specifications, use or the like of the semiconductordevice. In such a case, the electron concentration and the oxygencontent may be adjusted in consideration of the productivity thereof, sothat a compound semiconductor exhibiting an electron mobility of 30cm²/V·s or higher is manufactured and applied to a structural portion ofthe device for which such a compound semiconductor is needed.

According to the present invention, the pulse sputtering method (PSDmethod) is used to form a sputtered single crystal film with nohigh-temperature process. More preferably, a compound semiconductor filmis formed in a process performed generally at room temperature. There isno limit on the area size of the substrate, and films of various sizesfrom a small size to a large size may be manufactured.

For example, a compound semiconductor film of a rectangular outer shapehaving a length of a side of 2 inches or longer or a compoundsemiconductor film of a circular outer shape having a diameter of 2inches or longer may be formed. Alternatively, a compound semiconductorfilm having an area size that is 30 cm² or larger and having anallowable area within the restriction of the internal space of thesputtering apparatus may be formed.

In this case, a high quality compound semiconductor film is easilyformed with no need of a buffer layer, which is required by theconventional technology.

Now, the properties of the compound semiconductor according to thepresent invention will be described. The specific resistance ρ of ann-type nitride semiconductor film is in inverse proportion to theelectron mobility μ_(n) and the carrier concentration n. Therefore, then-type nitride semiconductor film exhibiting a high electron mobilityeven at a high electron concentration indicates that a high quality filmhaving a low electric resistance is formed. Namely, the presentinvention provides a high quality group 13 nitride semiconductor filmeasily usable for a semiconductor device. The compound semiconductoraccording to the present invention has a threading dislocation densityof about 1×10⁶ /cm² to about 5×10¹⁰ /cm².

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between the electronconcentration (N_(e)) and the electron mobility (μ_(e)) of a Si-dopedn-type GaN film produced by a PSD method.

FIGS. 2(a) and 2(b) provide graphs of SIMS data each showing a profile,in a depth direction, of the oxygen concentration of a GaN film having aSi concentration of 2×10²⁰ cm⁻³.

FIGS. 3(a) and 3(b) show AFM images of surfaces of the Si-doped GaNfilms shown in FIG. 2 formed by sputtering.

FIGS. 4(a) and 4(b) provide graphs respectively showing the absorptioncoefficient and the refractive index of a GaN film having a Siconcentration (electron concentration) of 2×10²⁰ cm⁻³ measured by anellipsometer.

FIGS. 5(A) and 5(B) provide a schematic view (A) and a schematic view ina planar direction (B) each showing a crystal structure of GaN.

FIG. 6 is a schematic view showing a structure of a sputtering apparatususable in the present invention.

FIG. 7 is a graph showing an example of pulse sequence to be applied toan electrode of the sputtering apparatus at the time of sputteringaccording to the present invention.

FIG. 8 is a schematic vertical cross-sectional view showing an innerstructure of a sputtering apparatus usable in the present invention.

FIG. 9 is a schematic cross-sectional view of a semiconductor deviceaccording to embodiment 1 of the present invention.

FIG. 10 is a schematic cross-sectional view showing a contact structureaccording to embodiment 2 of the present invention.

FIG. 11 is a schematic cross-sectional view showing a contact structureaccording to embodiment 3 of the present invention.

FIG. 12 is a schematic cross-sectional view of a thin film transistor towhich the present invention is applicable.

FIG. 13 is a schematic cross-sectional view of an AIGaN/GaN HEMT towhich the present invention is applicable.

FIG. 14 is a schematic cross-sectional view of an LED device to whichthe present invention is applicable.

FIG. 15 is a schematic cross-sectional view of a surface emitting laserdevice to which the present invention is applicable.

FIG. 16 shows the relationship between the electron concentration andthe resistivity of GaN according to the present invention.

FIG. 17 shows the relationship between the concentrations of donorimpurities and the electron concentrations, of the GaN according to thepresent invention, obtained by a SIMS measurement.

FIGS. 18(a), 18(b), 18(c), and 18(d) provide AFM images of surfaces ofGe-doped GaN samples as examples of surface state of GaN.

FIG. 19 is a schematic cross-sectional view of a vertical power MOSFET.

FIG. 20 is a schematic cross-sectional view of a GaN-based LED.

FIG. 21 is a schematic cross-sectional view of a Schottky diode.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a compound semiconductor manufactured by pulse-sputteringusing a group 13 nitride semiconductor will be described as anembodiment according to the present invention with reference to thedrawings.

A group 13 nitride semiconductor according to an embodiment of thepresent invention is formed as a film by a pulse sputter depositionmethod (PSD method).

Pulse Sputtering Method

The “pulse sputtering method (PSD method)” used to manufacture acompound semiconductor of a nitride according to the present invention,and the materials and the manufacturing method used to manufacture thecompound semiconductor, are basic items well known to a person ofordinary skill in the art.

For example, the standard technologies disclosed in the followingpublications are usable to work the present invention with no problem:“Growth of a nitride substrate and a lattice-matched substrate anddevice characteristics” (CMC Publishing Co., Ltd.; first editionpublished on Oct. 30, 2009), “New development of high frequencysemiconductor materials and devices” (CMC Publishing Co., Ltd.; firstedition published on Nov. 13, 2006), “Improvement in performance ofnext-generation power semiconductors and industrial development thereof”(CMC Publishing Co., Ltd.; first edition published on Jun. 10, 2015),Japanese Laid-Open Patent Publication No. 2009-138235 “Pulse sputteringapparatus, and pulse sputtering method”, Japanese Laid-Open PatentPublication No. 2014-159368 “Gallium nitride sintered body or galliumnitride molded article, and method for producing the same”, and thelike. Patent Documents 2 and 3, and Non-patent Documents 3 and 4, andthe like may also be referred to.

According to the PSD method used in the present invention, crystalgrowth advances based on a physical reaction, and therefore, may beperformed at a low temperature. In addition, carbon and hydrogen in afilm formation environment are conspicuously removable. Since thecrystal growth may be performed at a low temperature, generation of athermal stress in the film is suppressed, and also a compound easilycausing phase separation such as, for example, InGaN, is stably grown.

Single crystal growth of a compound semiconductor according to thepresent invention is not visually recognizable directly, but theprinciple of action of the crystal growth is generally considered asfollows. FIG. 5 shows a crystal structure of GaN, which is one oftwo-component group 13 compounds. During the film formation of acompound semiconductor according to the present invention, it isconsidered that a polar surface at which Ga atoms of GaN are located toform a hexagonal shape (Ga atom surface) is aligned to a surface of asubstrate acting as an underlying layer, so that a single crystalstructure is formed.

In this step, with the manufacturing method used in the presentinvention, the film formation is allowed to be performed at a relativelylow temperature, instead of at a high temperature exceeding 1000° C.required by the MOCVD method or the like. The temperature range to beused is 700° C. or lower and may include room temperature of 25° C.(room temperature to 700° C.). Although the temperature varies inaccordance with the film formation rate, a preferable temperature rangemay be, for example, 300 to 700° C.

For this reason, it is estimated that a small number of oxygen atomscontained in the film formation atmosphere are present to cover asurface of the film to be formed during the film formation. It isconsidered that as a result of the above, the oxygen atoms act toprevent the bonding of the group 13 element and nitrogen, and therefore,the film formation process advances while main elements to form thedesired compound are kept free.

In addition, it is considered that since the film formation conditionsare the same for the entirety of the underlying layer in a planardirection, a crystal structure that is uniform and has a high level ofcrystallinity entirely is formed.

The GaN compound semiconductor formed as a sputtered film in this manneris considered to gradually grow in an axial direction of the hexagonalshape (thickness direction of the film), so that in a final step, acompound semiconductor film that is uniform in the plane and has atleast a certain area size is manufactured.

It is preferred that the underlying layer to be used is formed of amaterial fulfilling the condition of having a lattice matched with, ormatched in a pseudo manner with, the compound semiconductor to be grown.The film formation process by the PSD method is not performed at a hightemperature exceeding 1000° C. Therefore, the material of the underlyinglayer does not need to be resistant against a high temperature. However,in order to improve the crystallinity, it is preferred that the crystaland the underlying layer fulfill the conditions of being lattice-matchedor pseudo-lattice-matched with each other.

For the above-described reasons, according to the present invention, itis especially preferred that the material of the underlying layer isselected from the four types: SiC, sapphire, GaN, single crystallinesilicon. Sapphire has a heat resistant temperature of 1200° C., andsingle crystalline silicon has a heat resistant temperature of 1100° C.These materials are usable to manufacture semiconductor devices such asAlGaN/GaN HEMTs, full-color LEDs, InGaN-TFTs, sensors and the like.

Alternatively, the material of the underlying layer may be, for example,metal foil or alkali-free glass for FPD having a heat resistanttemperature of 600 to 700° C., or the like, although the formed crystalquality of the compound semiconductor is inferior to the quality in thecase where the above-listed materials are used. In this case, it ispreferred that a buffer layer is formed on a surface of the material ofthe underlying layer for the crystal growth, for the purpose of makingthe underlying layer pseudo-lattice-matched with the compoundsemiconductor.

Regarding the size of the film to be formed according to the presentinvention, a device having a length of a shorter side or a diameter of acircle of 2 inches to 10 inches may be manufactured. The presentinvention is also applicable to a medium-sized device having a diagonalline of a rectangle of 10 to 30 inches and a large device having adiagonal line of a rectangle of 30 inches or longer. The device or thesubstrate acting as the underlying layer may be circular, square,rectangular, or of an asymmetrical shape.

FIG. 6 and FIG. 7 respectively show a schematic view of a sputteringapparatus and a pulse sequence usable to manufacture a compoundsemiconductor according to the present invention. A sputtering apparatus1 mainly includes a chamber 11, a substrate electrode 12, a targetelectrode 13, a DC power supply 14, a power supply controller 15, anitrogen supply source 16, a heating device and the like.

The chamber 11 is sealable against the outside. The inner pressure ofthe chamber 11 is allowed to be decreased by a vacuum pump or the like(not shown). The substrate electrode 12 is located in the chamber 11,and is capable of holding a heat dissipation sheet 12 a.

The target electrode 13 is provided in the chamber 11 so as to face thesubstrate electrode 12, and is capable of holding a target 13 a. Thetarget 13 a is formed of a compound of a group 13 element and nitrogen.A high quality target material with little impurities that is currentlyavailable in general is used. For example, a high quality material suchas the five-nine or six-nine level is needed.

The DC power supply 14 is electrically connected with the substrateelectrode 12 and the target electrode 13, and is a voltage source thatapplies a DC voltage between the substrate electrode 12 and the targetelectrode 13.

The power supply controller 15 is connected with the DC power supply 14,and performs control regarding the timing of the operation of the DCpower supply 14. The power supply controller 15 allows a pulse voltageto be applied between the substrate electrode 12 and the targetelectrode 13.

The nitrogen supply source 16 is connected with the inside of thechamber 11 by, a supply tube or the like, and supplies nitrogen gas intothe chamber 11. Although not shown, an argon gas supply source thatsupplies argon gas into the chamber is also provided in addition to thenitrogen gas supply source 16.

An oxygen supply source that supplies a predetermined amount of oxygenis also provided. The internal pressure is constantly allowed to bemonitored while the film is formed. The content of oxygen in the chamberneeds to be controlled to be kept at about 10 ppm substantiallyconstantly during the film formation of the compound semiconductor.

In order to realize this, it is indispensable that the chamber used forthe pulse sputtering, the supply system of the process gas and thedischarge system of the process gas (main discharger, rough discharger)prohibit gas leak and invasion of external air, and it is important thatthe pressure is controlled to be highly stable during the filmformation. It is considered to be fundamental to supply a trace amountof oxygen into the chamber intentionally. In order to realize this, thechamber needs to be confirmed to have been cleaned, and the materials tobe used need to have a high purity.

The heating device 17 is secured to, for example, the substrateelectrode 12, so that the temperature around the heat dissipation sheet12a on the substrate electrode 12 is adjustable. The representativeexamples of the film formation conditions to be used according to thepresent invention are as follows. FIG. 7 is an example of pulsesequence. The voltage P_(A) of the driving pulse is adjustable. The filmformation rate is generally 0.1 to 4 nm/sec. on average, and morepreferably 0.2 to 2 nm/sec.

-   -   (a) Driving method: pulse sputtering method (PSD method)    -   (b) Duty ratio: 5%    -   (c) Average power: 100 V    -   (d) Pulse frequency: 1 kHz    -   (e) Growth pressure: 2×10⁻³ Torr    -   (f) Dopant: Si

The film formation by the sputtering was performed in atmospheric gascontaining argon as a main component, and the substrate temperatureduring the film formation was set to the range of 300 to 700° C. In thiscase, doping gas such as SiH₄, GeH₄ or the like is usable as the dopingmaterial, and a target containing Si or Ge atoms is usable, in order toform a high concentration n-type group 13 nitride compoundsemiconductor.

Experiments were made in which oxygen were incorporated at aconcentration of 10 ppm into the atmospheric gas to be used for thesputtering in order to introduce oxygen into the film of the targetcompound semiconductor to be manufactured, and in which oxygen was notincorporated. Physical characteristics of the compound semiconductormanufactured with oxygen and the compound semiconductor manufacturedwith no oxygen were checked in comparison with each other.

FIG. 8 is a schematic vertical cross-sectional view of a continuous filmformation apparatus 10 of a roll-to-roll system. A plurality of filmformation chambers 5 are provided inside the continuous film formationdevice 10. The present invention is applicable to such a device as longas the substrate film 4 is a metal foil or a very thin film-like glasssubstrate that may be taken into a roll or taken out of a roll. Whilethe flexible substrate film 4 is transported in a horizontal directionfrom a take-out roll 2 to a take-in roll 3, the sputtering may beperformed toward the substrate film 4 at a plurality of locations in thefilm formation chamber. As a result, a semiconductor device containing adesired compound semiconductor or the like is processed at a high speed.The table in the chamber is usable for, for example, a diameter of 320to 600 mm.

According to the present invention, crystal growth of a compoundsemiconductor is realized on an underlying layer or a substrate havingan area size defined by a shorter side of a rectangle or a diameter ofat least 2 inches. The crystal is manufactured at a low temperature andat a high rate so as to have a certain area size and to be uniform. Inaddition, a novel compound semiconductor is mass-manufactured while themanufacturing cost thereof is suppressed.

FIG. 1 shows the relationship between the electron concentration (N_(e))and the electron mobility (μ_(e)) of a Si-doped n-type GaN film producedby the PSD method by the present inventors. The electron concentrationand the electron mobility were determined by a room temperature Halleffect measurement. The electron concentration (N_(e)) is considered tobe substantially equal to the Si donor concentration. The film formationby the sputtering was performed in atmospheric gas containing argon gasas a main component, and the substrate temperature during the filmformation was in the range of 300 to 700° C.

Oxygen was incorporated at a concentration of 10 ppm into theatmospheric gas to be used for the sputtering for the purpose ofintroducing oxygen into this film, so that a crystal film exhibitingsingle crystallinity was formed. An n-type ohmic electrode metal stackstructure (Ti (20 nm)/AI (60 nm)/Ti (20 nm)/Au (50 nm)) was formed on asurface of the resultant GaN thin film, and was annealed in nitrogen at700° C. The contact resistance of samples formed in this manner wasevaluated by a TLM method and was found to be 8.5×10⁻⁵ Ω·cm².

In this figure, the circles show the actually measured values, and thecurve shows the fitting result based on the Caughey-Thomas-typeempirical formula (formula 1 below; see Non-patent Document 4), which isused to describe the mobility in a low electric field. In the formulabelow, N_(D) is the donor concentration. Since the electronconcentration (N_(e)) is considered to be substantially equal to the Sidonor concentration as described above, the fitting is performed with anassumption that N_(D)=N_(e).

μ=μ_(min)+[μ_(max)−μ_(min)]/[1+(N_(D)/N_(R))^(Y)]  (1)

From the above-shown fitting result,

μ_(max)=1034 cm²/V·s,

μ_(min)=125 cm²/V·s

were found. These values are comparable to the highest value of themobility of the n-type GaN thin film formed by the MOCVD methodconventionally reported (see, for example, Non-patent Document 5). Ascan be seen, it has been confirmed that the carrier scattering issufficiently suppressed in the film of the compound semiconductormanufactured according to the present invention.

With the MOCVD method of the conventional technology, it is consideredto be difficult to form a GaN thin film exhibiting such a high electronmobility when the donor concentration is generally 5×10¹⁹ cm⁻³ orhigher. According to the present invention, as shown in FIG. 1, theSi-doped n-type GaN film produced by the PSD method exhibits the valuesmatching the Caughey-Thomas-type empirical formula even at the donorconcentration of at least 2×10²⁰ cm⁻³.

Namely, it has been found out that an n-type GaN film according to thisexample produced by the PSD method is a very high quality filmexhibiting an electron mobility of 46 cm²/V·s or higher even at anelectron concentration of 5×10¹⁹ cm⁻³ or higher. Preferably, a filmexhibiting an electron mobility of 50 cm²/V·s or higher is usable. Thespecific resistance p of an n-type nitride semiconductor film is ininverse proportion to the electron mobility μ_(n) and the carrierconcentration n. Therefore, the n-type nitride semiconductor filmexhibiting a high electron mobility even at a high electronconcentration indicates that a high quality film having a low resistanceis formed.

The samples shown in FIG. 1 are all Si-doped. The impurity to beincorporated as a donor is not limited to Si and may be Ge or the like.

When the donor concentration of the nitride semiconductor film isincreased in order to realize a high electron concentration, thetransparency of the film to visible light is decreased. This causes aconcern that an inconvenience may occur in the case where the nitridesemiconductor film according to the present invention is used for atransparent electrode or the like.

Under such circumstances, according to the present invention, thedecrease in the transparency caused by the increase in the electronconcentration of the film of the compound semiconductor is compensatedfor as follows. The nitrogen site is replaced, so that oxygen, which isa dopant acting as a donor, is incorporated as an impurity to expand thebandgap of the film.

The bandgap of an oxygen-doped film depends on the amount of doping. Forexample, in the case of GaN, the bandgap at room temperature may bevaried in the range of 3.4 eV to 4.9 eV (value of the bandgap of galliumoxide). In the case of, for example, GaN, when oxygen is incorporated asan impurity at 1×10¹⁷ cm⁻³ or higher into the film, the bandgap at roomtemperature is generally about 3.4 to about 3.6 eV.

Such an effect of oxygen, for example, allows the nitride semiconductorfilm according to this example to have an absorption coefficient of 2000cm⁻¹ or less to light having a wavelength region of 405 nm or to have anabsorption coefficient of 1000 cm⁻¹ or less to light having a wavelengthregion of 450 nm. In this manner, the nitride semiconductor filmaccording to this example is usable for a transparent electrode with noinconvenience.

FIG. 2 provides graphs each showing an oxygen concentration of the GaNfilm according to this embodiment manufactured by the PSD method. FIG.2(b) shows SIMS data representing a profile, in a depth direction, ofthe oxygen concentration of a GaN film having a Si concentration of2×10²⁰ cm⁻³, among the samples shown in FIG. 1. It is understood thatthe oxygen is contained at a concentration of about 1 to 3×10¹⁸ cm⁻³.This film exhibits an electron mobility of 110 cm²/V·s.

The RMS value of an AFM image representing the surface roughness of thisfilm was 3.97 nm as seen from FIG. 3(b). The present inventors performedan AFM measurement on the film samples formed by the present inventorsat various electron concentrations and containing oxygen at an electronconcentration of 5×10¹⁹ cm⁻³ or higher. All the samples had an RMS valueof 5.0 nm or less.

In the meantime, crystal growth was performed under substantially thesame conditions but with no incorporation of 10 ppm oxygen into theatmospheric gas. The results were as follows. As shown in the profile inFIG. 2(a), the oxygen concentration was about 1×10¹⁶ cm⁻³, and themobility at this point was 45 cm²/V·s. As seen from FIG. 3(a), the RMSvalue representing the surface roughness of this thin film was 14.1 nm.

Now, the two conditions, namely, the condition of incorporating oxygenand the condition of not incorporating oxygen, will be discussed. In thecase with oxygen, it is considered that oxygen atoms in the atmospherecovering a surface of the film that is being formed cause the stress toalleviate and the migration of the atoms at the surface to promote. Itis considered that this suppression on the surface roughness suppressesintroduction of point defects and thus improves the mobility. At a hightemperature used by the MOCVD method or the like of the conventionaltechnology, oxygen evaporates from the surface. Therefore, it isconsidered to be difficult to provide the effect of improving thequality realized by the low-temperature growth performed by the PSDmethod.

By contrast, in the case with no oxygen, it is considered that theabove-described action is not easily provided and thus the crystal ofthe film formed by the PSD method is likely to include defects.

FIG. 4 provides graphs showing the absorption coefficient (FIG. 4(a))and the refractive index (FIG. 4(b)) of a GaN film having a Siconcentration (electron concentration) of 2×10²⁰ cm⁻³ measured by anellipsometer. This film exhibits an electron mobility of 115 cm²V·s.This film has an absorption coefficient of 844 cm⁻¹ at a wavelength of450 nm, which is used for a blue LED as a standard wavelength, and hasan absorption coefficient of 1860 cm⁻¹ at a wavelength of 405 nm, whichis used for a blue-violet laser as a standard wavelength.

As can be seen, the oxygen doping allows the film to have an absorptioncoefficient of 2000 cm⁻¹ or less to light having a wavelength region of405 nm or to have an absorption coefficient of 1000 cm⁻¹ or less tolight having a wavelength region of 450 nm. As a result, the obtainedcompound semiconductor is usable as a transparent material.

Hereinafter, various forms of electronic device to which a compoundsemiconductor according to the present invention is applicable.

EMBODIMENT 1

FIG. 9 is a schematic cross-sectional view of a compound semiconductordevice 20 including a group 13 nitride semiconductor formed on asubstrate. Reference sign 21 represents the substrate (sapphire), andreference sign 22 represents GaN.

EMBODIMENT 2

FIG. 10 is a schematic cross-sectional view of a contact structureformed using a compound semiconductor according to the presentinvention. Reference sign 31 represents a GaN substrate, reference sign32 represents GaN (film of a compound semiconductor formed by the PSDmethod), reference sign 34 represents an insulating film, reference sign33 represents a wiring electrode connectable with an external device,and reference sign 35 represents a contact hole.

EMBODIMENT 3

FIG. 11 is a schematic cross-sectional view of a contact structure 40formed using a group 13 nitride compound semiconductor according to thepresent invention. In FIG. 11, reference sign 41 represents an n-typeGaN contact layer, reference sign 42 represents a Ti layer, referencesign 43 represents an Al layer, reference sign 44 represents an Nilayer, and reference sign 45 represents an Au layer. In this example, acomposite metal electrode is used. After the film formation, heattreatment is performed at about 900° C.

APPLICATION EXAMPLES

FIG. 12 is a schematic cross-sectional view of a thin film transistor towhich the present invention is applicable. A high concentration n-typeGaN layer is applicable as a contact layer of an electrode of the thinfilm transistor.

In the figure, reference sign 51 represents a substrate formed ofalkali-free glass or the like, reference sign 52 represents aninterlayer insulating layer, reference sign 53S represents a source-sidecontact layer (high concentration n+GaN layer), reference sign 54Srepresents a source region, reference sign 55 represents an activelayer, reference sign 54D represents a drain region, reference sign 53Drepresents a drain-side contact layer (high concentration n⁺GaN layer),reference sign 56 represents a gate oxide film, reference sign 57represents a source electrode, reference sign 58 represents a gateelectrode, and reference sign 59 represents a drain electrode. Thesource region 54S and the drain region 54D are each formed such that theconcentration of the impurity is gradually changed between thecorresponding contact layer and the active layer.

FIG. 13 is a schematic cross-sectional view of a HEMT to which thepresent invention is applicable. A high concentration n-type GaN layeraccording to the present invention is applicable as contact layerslocated below, and in contact with, source and drain electrodes of theHEMT of AIGaN/GaN. In the figure, reference sign 61 represents asubstrate formed of GaN, sapphire, SiC, Si or the like, reference sign62 represents a buffer layer formed of GaN, AIN or the like, referencesign 63 represents a GaN undoped layer, reference sign 64 represents anAIGaN barrier layer, and reference sign 65 represents a contact layerformed using a high concentration n-type GaN layer. A source electrode66, a gate electrode 67 and a drain electrode 68 are provided in a toppart of the HEMT.

In the thin film transistor (FIG. 12) and the HEMT (FIG. 13) describedabove, the high concentration n-type GaN layer is applicable as thecontact layer. The contact resistance of such a contact layer against anelectrode in a circuit element in which an operating current flows (thecircuit element is each of source and drain in these devices) issignificantly decreased. This significantly contributes to theimprovement in performance of the electronic device.

FIG. 14 is a schematic cross-sectional view of an LED device as anexample of GaN-based semiconductor device to which the present inventionis applicable.

As shown in this figure, a plurality of compound semiconductor layersare sequentially stacked from the side of a substrate 71 formed of GaN,sapphire, SiC or Si. A buffer layer 72, an n-type GaN layer 73, aGaInN/GaN MQW light emitting layer 74, a p-type GaN layer 75, a tunneljunction 76 including a p-type InGaN layer 76 a and a high concentrationn-type GaN layer 76 b, an n-type GaN layer 77, a contact layer 78 formedof a high concentration n-type GaN layer, and electrodes 79A and 79B areprovided.

FIG. 15 is a schematic cross-sectional view of an InGaN/GaN VCSEL(surface emitting laser) structure to which the present invention isapplicable. In such a vertical cavity surface emitting laser (VCSEL), aresonator is formed to be perpendicular to a surface of a semiconductorsubstrate. Therefore, laser light is output perpendicularly to thesubstrate surface.

In the figure, reference sign 81 represents a GaN substrate, referencesign 82D represents an inner multi-layer reflection mirror, referencesign 83 represents an n-type GaN layer, reference sign 84 represents anMQW active layer formed of GalnN/GaN, reference sign 85 represents ap-type AIGaN layer, reference sign 86 a represents a p-type InGaN layer,and reference sign 86 b represents a high concentration n-type GaNlayer. 86 a and 86 b form a tunnel junction 86. Reference sign 87represents an n-type GaN layer, reference sign 88 represents a highconcentration n-type GaN layer (contact layer), reference sign 89A and89B represent electrodes, and reference sign 82U represents an uppermulti-layer reflection mirror.

As described above, the compound semiconductor according to the presentinvention is usable for, for example, regions of a light emitting deviceor an electronic device in which a large amount of electric currentflows, a contact portion of a semiconductor device, or an electrodestructure such as a transparent electrode or the like. The compoundsemiconductor according to the present invention is preferably usablefor a wire or the like of an electronic device drivable at a very lowvoltage. The compound semiconductor according to the present inventionis adaptable to the specifications of large electric current and largeelectric power, which are not easily dealt with by the conventionaltechnology.

The compound semiconductor according to the present invention exhibits ahigh electron mobility and thus has a low resistance, and therefore isconsidered to contribute to improvement in the operation speed ofdevices.

So far, a compound semiconductor according to the present invention,namely, a two-, three- or four-component compound semiconductor thatcontains nitrogen and one element selected from the group consisting ofB, Al, Ga and In, which are group 13 elements, contains oxygen as animpurity at 1×10¹⁷ cm⁻³ or higher, has an electron concentration of5×10¹⁹ cm⁻³ or higher, has n-type conductivity and exhibits an electronmobility of 46 cm²/V·S or higher has been described.

Hereinafter, a nitride semiconductor according to a second inventionmade by the present inventors will be described.

The nitride semiconductor has a conspicuous feature of exhibiting alower specific resistance (namely, exhibiting a higher mobility) than aconventional semiconductor although being in the form of a crystal dopedwith a donor at a high concentration.

Specifically, the nitride semiconductor contains nitrogen and at leastone group 13 element selected from the group consisting of B, Al, Ga andIn, has n-type conductivity, exhibits an electron concentration of1×10²⁰ cm⁻³ or higher, and exhibits a specific resistance of 0.3×10⁻³Ω·cm or lower. Preferably, the at least one group 13 element is Ga, andeither one of, or both of, Si and Ge are contained as donor impurities.

Conventionally, a nitride semiconductor doped with Ge grown by the MBEmethod at a high concentration and exhibiting a relatively low specificresistance is known. As compared with such a nitride semiconductor, thenitride semiconductor according to the present invention realizes alower specific resistance in a region having a higher electronconcentration.

Such a nitride semiconductor exhibiting a low specific resistance(exhibiting a high mobility) although being in the form of a crystaldoped with donors at a high concentration is expected to be used forvarious uses, for example, to decrease the parasitic resistance of anelectronic device such as a HEMT or the like, to provide a materialreplacing a transparent conductive film of ITO or the like, and torealize cascade connection of LED modules.

FIG. 16 shows the relationship between the electron concentration (cm⁻³)and the resistivity (mΩ·cm) of GaN according to the present invention.In the figure, star marks represent the GaN according to the presentinvention. Among the star marks, white star marks represent Si-dopedGaN, and gray star marks represent Ge-doped GaN. The figure also shows,for comparison, data of GaN obtained by the MOCVD method (diamond-shapedmarks) and the MBE method (circular marks) reported so far, and alsoshows the relationship between the electron concentration and theresistivity obtained by a theoretical calculation. In the figure, θrepresents the compensation ratio of the concentration of ionizedimpurities (ratio of the acceptor concentration NA and the donorconcentration N_(D); N_(A)/N_(D)).

The GaN crystal conventionally reported exhibits a tendency that thespecific resistance is decreased as the electron concentration isincreased regardless of whether the crystal is obtained by the MBEmethod or the MOCVD method. However, the specific resistance isincreased when the electron concentration is above a certain level.

For example, in the case of GaN obtained by the MOCVD method, Si-dopedGaN shows an increase in the specific resistance from when the electronconcentration exceeds about 5×10¹⁹ cm⁻³, and Ge-doped GaN shows anincrease in the specific resistance from when the electron concentrationexceeds about 1×10²⁰ cm⁻³. In the case of GaN obtained by the MBEmethod, Si-doped GaN shows an increase in the specific resistance fromwhen the electron concentration exceeds about 1.5×10²⁰ cm⁻³, andGe-doped GaN shows an increase in the specific resistance from when theelectron concentration exceeds about 5×10²⁰ cm⁻³.

By contrast, in the case of GaN according to the present invention,neither Si-doped GaN (white marks) nor Ge-doped GaN (gray marks) showsany such increase in the specific resistance even when the electronconcentration is 5×10²⁰ cm⁻³.

In addition, in the case of the conventional GaN, even Ge-doped GaN,obtained by the MBE method and exhibiting the lowest specific resistancein a region of a high electron concentration, exhibits a specificresistance of merely 0.4 mΩ·cm (0.4×10⁻³ Ω·cm) at the minimum at anelectron concentration of about 5 ×10²⁰ cm⁻³. By contrast, the GaNaccording to the present invention exhibits a specific resistance of 0.2mΩ·cm (0.2×10⁻³ Ω·cm) at generally the same electron concentration.

As is clear from the results shown in this figure, unlike theconventional GaN, the GaN according to the present invention has afeature of exhibiting a conspicuously low specific resistance of0.3×10⁻³ Ω·cm or lower especially when the electron concentration is1×10²⁰ cm⁻³ or higher, and this feature is not lost even when theelectron concentration is 2×10²⁰ cm⁻³ or higher. As shown in the tablebelow, this tendency has been experimentally confirmed in the range ofspecific resistance down to 0.196×10⁻³ Ω·cm. The theoretical value ofthe lowest limit of the resistance value caused by scattering of ionizedimpurities is 0.04×10⁻³ Ω·cm, but is varied to, for example, 0.2×10⁻³Ω·cm, 0.15×10⁻³ Ω·cm, 0.1×10⁻³ Ω·cm or the like depending on the filmformation conditions or the like.

FIG. 17 shows the relationship between the concentration of the donorimpurities and the electron concentrations, of the GaN according to thepresent invention, obtained by a SIMS measurement. It is understood fromthese results that the activation ratio of the donors is about 1 in theGaN according to the present invention obtained by the PSD method.Namely, it is understood that for the GaN according to the presentinvention, the electron concentration is controllable by merelycontrolling the doping concentration of the donor impurity.

The various characteristics (electron concentration, electron mobility,specific resistance, and surface roughness) of the GaN according to thepresent invention are shown in Table 1 (Si-doped GaN) and Table 2(Ge-doped GaN).

TABLE 1 Si-doped GaN Electron Electron Specific Surface concentrationmobility resistance roughness ( cm⁻³) (cm² V⁻¹ s⁻¹) (m Ω · cm) RMS value(nm) 1.12E+19 211 2.64 0.85 2.16E+19 159 1.82 0.95 3.02E+19 154 1.340.94 4.75E+19 150 0.876 0.70 8.09E+19 136 0.567 0.90 9.36E+19 128 0.5210.88 1.44E+20 126 0.344 0.65 1.47E+20 126 0.337 0.75 1.66E+20 115 0.3270.55 1.93E+20 106 0.305 0.88 1.99E+20 110 0.285 0.95 2.03E+20 110 0.2790.68 2.95E+20 108 0.196 0.76

TABLE 2 Ge-doped GaN Electron Electron Specific Surface concentrationmobility resistance roughness (cm⁻³) (cm² V⁻¹ s⁻¹) (m Ω · cm) RMS value(nm) 1.24E+19 153 3.29 0.91 2.03E+20 96.8 0.280 0.77 2.87E+20 82.1 0.2650.62 3.04E+20 79.0 0.260 0.62 3.24E+20 74.6 0.258 0.54 3.28E+20 77.40.246 0.46 3.36E+20 73.8 0.252 0.46 3.39E+20 70.2 0.262 0.31 3.54E+2072.2 0.244 0.34 3.99E+20 73 0.214 0.35 4.11E+20 70.4 0.216 0.65 4.35E+2070.9 0.202 0.65 4.49E+20 66.2 0.210 0.55 4.70E+20 66.2 0.200 0.555.15E+20 60.1 0.202 0.86 5.25E+20 57.8 0.207 0.86 5.49E+20 41.3 0.2750.86

The GaN shown in Table 1 and Table 2 is all obtained in generally thesame conditions as the crystal growth conditions by the PSD methoddescribed above. The materials and the like each having the followingpurity were used. The electron concentration was changed by changing thepower applied to the cathode from 20 to 150 W.

Substrate temperature during the growth: 600 to 700° C.

Sputtering target (Si): single crystal having a purity of 99.999%

Sputtering target (Ge): single crystal having a purity of 99.99%

Ga: Purity: 99.99999%

Nitrogen gas: purity: 99.9999%

The present inventors note that the vacuum level of the film formationenvironment and the quality of the vacuum state are important forgrowing a high quality crystal, and appropriately adjusted theconditions of pulse sputtering (pulse voltage, pulse width, duty ratio,etc.) in order to obtain a crystal of a desired film quality. It is oneof advantages of the PSD method that such fine adjustments may be madequickly.

The measurement conditions and the like for the above-mentioned variousproperties are as follows.

The electron concentration and the electron mobility were measured byuse of a Hall measurement device (ResiTest8400, Toyo Corporation) whilethe applied current was varied in the range of 1 mA to 10 mA and theapplied magnetic field was varied in the range of 0.1 to 0.5 T (tesla)in accordance with the resistivity of the sample. The temperature forthe measurement was room temperature.

The surface roughness was measured by use of an AFM device (JSPM4200produced by JEOL Ltd.).

FIG. 18 shows AFM images of surfaces of the Ge-doped GaN samples asexamples of surface state of the above-described GaN. These samples allhave an RMS value less than 1 nm. In general, a surface having an RMSvalue, obtained by a surface roughness measurement by an AFM device, of5.0 nm or less may be evaluated to be sufficiently flat. Inconsideration of this, it is understood that the nitride semiconductoraccording to the present invention has a highly flat surface.

Nitride semiconductor crystals having the Ga site of GaN be partiallyreplaced with Al or In (AIGaN or InGaN) were also produced, and variousproperties thereof were examined. The results are shown in Table 3 andTable 4. These samples each have an Al concentration of 1% and an Inconcentration of 1%. The purity and the like of each of the materialsused for the crystal growth are as follows.

Substrate temperature during the growth: 600 to 700° C.

Sputtering target (Si): single crystal having a purity of 99.999%

Sputtering target (Ge): single crystal having a purity of 99.99%

Ga: Purity: 99.99999%

Al: Purity: 99.999%

In: Purity: 99.999%

Nitrogen gas: purity: 99.9999%

TABLE 3 Ge-doped AlGaN Electron concentration Electron mobility Specificresistance (cm⁻³) (cm² V⁻¹ s⁻¹) (m Ω · cm) 476E+20 61.7 0.213

TABLE 4 Si-doped InGaN Electron concentration Electron mobility Specificresistance 4.76(cm⁻³) (cm² V⁻¹ s⁻¹) (m Ω · cm) 2.32E+20 98.4 0.273

The contact resistance of each of the nitride semiconductors shown inTable 1 to Table 4 was measured. It has been confirmed that all thesamples have a contact resistance of 1×10⁻⁴ Ω·cm² or less against ann-type ohmic electrode metal. Such a value is sufficiently low. Acontact structure including any of the above-described nitridesemiconductors for a conductive portion is expected to be used invarious uses, for example, to decrease the parasitic resistance of anelectronic device such as a HEMT or the like, to provide a materialreplacing a transparent conductive film of ITO or the like, and torealize cascade connection of an LED module.

The contact resistance was measured by use of a TLM (Transmission LineModel) measurement apparatus (semiconductor parameter analyzer Agilent4155C) on a TLM pattern including Ti/Al/Ti/Au electrode structures (100μm×100 μm) located at an inter-electrode distance of 2 μm to 100 μm.

As described above, the nitrogen site of the nitride semiconductor maybe replaced, so that oxygen, which is a dopant acting as a donor, isincorporated as an impurity to expand the bandgap of the film. In thismanner, the decrease in the transparency caused by the increase in theelectron concentration of the film of the nitride semiconductor iscompensated for.

For this purpose, for example, oxygen as an impurity is incorporated at1×10¹⁷ cm⁻³ or higher into the above-described nitride semiconductor.Such incorporation of oxygen as an impurity allows the nitridesemiconductor to have an absorption coefficient of 2000 cm⁻¹ or less tolight having a wavelength region of 405 nm or to have an absorptioncoefficient of 1000 cm⁻¹ or less to light having a wavelength region of450 nm.

The above-described nitride semiconductor according to the presentinvention is formed by the PSD method. The present inventors considerthat the above-described characteristics are obtained for the followingreason: with the other crystal growth methods, the crystal growthadvances in a thermal equilibrium state, whereas with the PSD method,the crystal growth advances in a thermal non-equilibrium state.

A nitride semiconductor such as GaN or the like doped with a donor at ahigh concentration is thermodynamically unstable, and therefore, ispartially decomposed even while the crystal growth is advancing. Namely,the growth and the decomposition of the crystal occur at the same time.Therefore, the donor impurities once incorporated into the crystal arepushed out at the time of decomposition. When it is attempted to dopethe nitride semiconductor with donor impurities at a high concentration,this phenomenon that the donor impurities are pushed out reaches to anunignorable level, and as a result, the crystallinity itself isdecreased. Namely, in the case where the nitride semiconductor is dopedwith the donor impurities at a high concentration, the decrease in thecrystallinity is unavoidable under the crystal growth conditions closeto the thermal equilibrium state.

By contrast, with the PSD method, the crystal growth advances in athermal non-equilibrium state. Therefore, the donor impurity is noteasily pushed out, and thus the crystallinity is not easily decreased.

In general, the Ge donor tends to be more easily incorporated into thenitride semiconductor crystal at a high concentration than the Si donor.One conceivable reason for this is the following. Since the radius ofthe Ge ion is close to the radius of Ga ion, the Ge ion easily replacesthe Ga ion site. As a result, the accumulation of stress in the nitridesemiconductor film is alleviated, and thus the surface of the film tendsto be flat.

As described above, the nitride semiconductor according to the presentinvention realizes a lower specific resistance in a region of a higherelectron concentration than the conventional nitride semiconductor.

There are the following documents that disclose inventions relating to anitride semiconductor device having a low on-resistance.

Japanese Laid-Open Patent Publication No. 2016-115931 (Patent Document4) discloses an invention relating to a nitride semiconductor devicehaving a low on-resistance. Paragraph 0049 describes that “as describedabove, the source-side nitride semiconductor regrowth layer 205 a andthe drain-side nitride semiconductor regrowth layer 206 a each maycontain n-type impurities at a high concentration. However, as shown inFIG. 4, when the impurity is silicon (Si), even if an impurity amount tobe supplied during the growth of a nitride semiconductor layer isincreased, the carrier concentration of the impurity in the nitridesemiconductor layer to be formed is not increased. That is, the impuritycarrier concentration has a certain upper limit. On the other hand, whengermanium (Ge) is used as the impurity, a higher carrier concentrationthan that of silicon can be realized”.

Paragraph 0095 describes that “in order to investigate thecharacteristics of the composite electrode of the nitride semiconductordevice 200 thus formed, the sheet resistance of the nitridesemiconductor regrowth layer itself and the contact resistance thereofwith the 2DEG were measured by a transmission line measurement (TLM)method. FIG. 7 shows the relationship between the sheet resistance ofthe nitride semiconductor regrowth layer itself and the supply amount ofGe. It was found that when the flow rate ratio of TEGe to TMG isincreased to 0.09 or more with an increase in supply amount of TEGe, anitride semiconductor regrowth layer having a lowered sheet resistanceof approximately 1.5×10⁻⁶ Ω·cm can be obtained. It was found that when anitride semiconductor regrowth layer formed under the conditionsdescribed above is used, the nitride semiconductor device 200 has acontact resistance of 1 to 5×10⁻⁶ Ω·cm, and a preferable contact withthe 2DEG can be obtained”.

Patent Document 4 and the corresponding United States Patent ApplicationPublication US2016/0172473 (Patent Document 5) filed claiming thebenefit of priority to Patent Document 4 were compared against eachother regarding the above description. As a result, it has been foundout that the name and the unit of the vertical axis of FIG. 7 arevariously changed. It is presumed that Patent Document 4 includes sometypographical error.

A technological document written by the inventors of Patent Document 4(IEDM14: Non-patent Document 9), pp. 275-278 (“Extremely lowon-resistance Enhancement-mode GaN-based HFET using Ge-doped regrowthtechnique”) will be referred to. This document discloses a Ge-dopednitride semiconductor regrowth layer exhibiting a low on-resistance.FIG. 3 is exactly the same as FIG. 7 of Patent Document 4.

The vertical axis is labeled as “Specific contact resistance (Ω·cm²)”.Regarding FIG. 3, there is a description “the measured specific contactresistance as a function of TEGe supply is shown in FIG. 3, whereextremely low specific contact resistance of 1.5×10⁻⁶ Ω·cm² wasachieved”. For this reason, it is considered that the vertical axis ofFIG. 7 of Patent Document 4 should be “contact resistance” and the unitshould be “Ω·cm²”.

If, as shown in FIG. 7 of Patent Document 4, the specific resistance isabout 1.5×10⁻⁶ Ω·cm and the Ge concentration (electron concentration) is1×10²⁰ cm⁻³, the electron mobility is about 42,000 cm²/V·s. This valueis far from the normal value known as the electron mobility of GaNcrystal (about 1,200 cm²/V·s). Based on this also, it is obvious thatthe above-described portion includes typographical errors.

As described above, Patent Document 4 is considered to disclose a“nitride semiconductor regrowth layer having a lowered contactresistance of approximately 1.5×10⁻⁶ Ω·cm²”.

The above-described nitride semiconductor according to the presentinvention has a feature of exhibiting a low specific resistance(exhibiting a high mobility) although being in the form of a crystaldoped with donors at a high concentration, and utilizing such a feature,is expected to be used for various uses, for example, to decrease theparasitic resistance of an electronic device such as a HEMT or the like,to provide a material replacing a transparent conductive film of ITO orthe like, and to realize cascade connection of an LED module. Forexample, the nitride semiconductor according to the present inventionmay be applied as follows.

Application to a Vertical Power MOSFET

FIG. 19 is a schematic cross-sectional view of a vertical power MOSFET.This vertical power MOSFET 100 includes an n⁺-GaN layer 105 of a nitridesemiconductor, according to the present invention, formed on a stackstructure including an n⁺-GaN layer 102, an n⁻-GaN layer 103 and a p-GaNlayer 104. The n⁺-GaN layer 105 according to the present invention maybe patterned as follows. After being deposited on the entire surface,the n⁺-GaN layer is patterned by lithography. Alternatively, a selectivegrowth technology may be used, according to which, a crystal surface ofgallium nitride is exposed to only a part of a surface of the sample,and the n⁺-GaN layer is epitaxially grown selectively on the exposedpart. Reference sign 106 represents an insulating film, reference sign101 represents a drain, reference sign 107 represents a source, andreference sign 108 represents a gate.

Application to an LED

FIG. 20 is a schematic cross-sectional view of a GaN-based LED. The LED200 includes an n-type nitride semiconductor layer 202, an active layer203 including a quantum well layer, a p-type nitride semiconductor layer204, and an n⁺-GaN layer 205 according to the present inventionsequentially stacked on a substrate 201 formed of a nitridesemiconductor.

A cathode electrode 206 is formed on a region of the n-type nitridesemiconductor layer 202 that is exposed as a result of the n⁺-GaN layer205, the p-type nitride semiconductor layer 204 and the active layer 203being partially removed. An anode electrode 207 is formed above thep-type nitride semiconductor layer 204 with the n⁺-GaN layer 205 beinglocated therebetween. The n⁺-GaN layer 205 according to the presentinvention is conductive with the p-type nitride semiconductor layer 204via a tunnel junction.

Application to a Schottky Diode

FIG. 21 is a schematic cross-sectional view of a Schottky diode. In thisSchottky diode 300, an n⁺-GaN substrate 301 has an n⁺-GaN layer 306according to the present invention formed on a rear surface thereof. Ann⁻-GaN layer 302 is formed on a front surface of the n⁺-GaN substrate301. An ohmic electrode 303 is formed on the n⁺-GaN layer 306 side, anda Schottky electrode 304 is formed on the n⁻-GaN layer 302 side. In thefigure, reference sign 305 represents an insulating film.

The nitride semiconductor according to the present invention exhibitinga low specific resistance (exhibiting a high mobility) although being inthe form of a crystal doped with a donor at a high concentration isusable for an n⁺-GaN layer of, for example, an IGBT (Insulated GateBipolar Transistor) in addition to the above-described devices.

As described above, the compound semiconductor according to the secondinvention made by the present inventors may be summarized as follows.

The second invention is directed to a nitride semiconductor havingn-type conductivity and containing nitrogen and at least one group 13element selected from the group consisting of B, Al, Ga and In, in whichthe nitride semiconductor has an electron concentration of 1×10²⁰ cm⁻³or higher and exhibits a specific resistance of 0.3×10⁻³ Ω·cm or lower.

Preferably, the electron concentration is 2×10²⁰ cm⁻³ or higher.

Preferably, the nitride semiconductor has a contact resistance of 1×10⁻⁴Ω·cm² or lower against an n-type ohmic electrode metal.

In an embodiment, the nitride semiconductor contains oxygen as animpurity at 1×10¹⁷ cm⁻³ or higher.

Preferably, the nitride semiconductor has an absorption coefficient of2000 cm⁻¹ or lower to light having a wavelength region of 405 nm.Preferably, the nitride semiconductor has an absorption coefficient of1000 cm⁻1 or lower to light having a wavelength region of 450 nm.

Preferably, the nitride semiconductor has an RMS value of 5.0 nm or lessobtained by a surface roughness measurement performed by an AFM.

In an embodiment, the at least one group 13 element is Ga.

In an embodiment, the nitride semiconductor contains either one of, orboth of, Si and Ge as donor impurities.

The lower limit of the specific resistance is, for example, 0.2×10⁻³Ω·cm, 0.15×10⁻³ Ω·cm, or 0.1×10⁻³ Ω·cm.

The relationship between the electron concentration and the specificresistance of the nitride semiconductor fulfills a numerical rangeenclosed by four points at which (a) the electron concentration is1×10²⁰ cm⁻³ and the specific resistance is 0.3×10⁻³ Ω·cm, (b) theelectron concentration is 3×10²⁰ cm⁻³ and the specific resistance is0.3×10⁻³ Ω·cm, (c) the electron concentration is 4×10²⁰ cm⁻³ and thespecific resistance is 0.15×10⁻³ Ω·cm, and (d) the electronconcentration is 9×10²⁰ cm⁻³ and the specific resistance is 0.15×10⁻³Ω·cm.

The above-described invention is applicable to a contact structure,comprising the nitride semiconductor for a conductive portion. Theabove-described invention is also applicable to a contact structure,comprising the nitride semiconductor for an electrode. Such a contactstructure is usable in a semiconductor device.

INDUSTRIAL APPLICABILITY

The two-, three- or four-component nitride semiconductor according tothe present invention exhibits an electron mobility of 46 cm²/V·s orhigher even in a region of a high electron concentration of 5×10¹⁹ cm⁻³or higher.

The present invention is applicable to an important circuit element thatdetermines the performance of an electronic circuit, such as a contactportion of a wiring structure that is included in an electronic devicehaving a low electric resistance and requiring a large amount ofelectric current, for example, a horizontal power semiconductor devicesuch as an HEMT or the like, a vertical power semiconductor device, ahigh withstand voltage diode, a thin film transistor, a display deviceor the like, an active layer or the like.

The nitride semiconductor according to the present invention is usablefor a high speed communication device, a computation device, a solarcell, a control circuit, an electronic device for an automobile or thelike in addition to the power semiconductor device, the display deviceand the light emitting device.

REFERENCE SIGNS LIST

1 Sputtering apparatus

2 Take-out roll

3 Take-in roll

4 Substrate film

5 Film formation chamber

10 Continuous film formation apparatus

11 Chamber

12 Substrate electrode

13 Target substrate

14 DC power supply

15 Power supply controller

16 Nitrogen supply source

17 Heating device

12 a Heat dissipation sheet

21 Substrate

22 GaN

31 Substrate

32 GaN

33 Insulating layer

34 Insulating layer

35 Contact hole

41 n-type GaN contact layer

42 Ti layer

43 Al layer

44 Ni layer

45 Au layer

100 Vertical power MOSFET

101 Drain

102 n⁺-GaN layer

103 n⁻-GaN layer

104 p-GaN layer

105 n⁺-GaN layer

106 Insulating film

107 Source

108 Gate

200 LED

201 Substrate

202 n-type nitride semiconductor layer

203 Active layer

204 p-type nitride semiconductor layer

205 n-side electrode

206 p-side electrode

300 Schottky diode

301 n⁺-GaN layer

302 n⁻-GaN layer

303 Ohmic electrode

304 Schottky electrode

305 Insulating film

306 n⁺-GaN layer

1. A nitride semiconductor having n-type conductivity and containingnitrogen and at least one group 13 element selected from the groupconsisting of B, Al, Ga and In, wherein the nitride semiconductor has anelectron concentration of 1×10²⁰ cm⁻³ or higher and exhibits a specificresistance of 0.3×10⁻³ Ω·cm or lower.
 2. The nitride semiconductoraccording to claim 1, wherein the electron concentration is 2×10²⁰cm^(−3 or) higher.
 3. The nitride semiconductor according to claim 1,wherein the nitride semiconductor has a contact resistance of 1×10 ⁻⁴Ω·cm² or lower against an n-type ohmic electrode metal.
 4. The nitridesemiconductor according to claim 1, wherein the nitride semiconductorcontains oxygen as an impurity at 1×10¹⁷ cm⁻³ or higher.
 5. The nitridesemiconductor according to claim 4, wherein the nitride semiconductorhas an absorption coefficient of 2000 cm⁻¹ or lower to light having awavelength region of 405 nm.
 6. The nitride semiconductor according toclaim 4, wherein the nitride semiconductor has an absorption coefficientof 1000 cm⁻¹ or lower to light having a wavelength region of 450 nm. 7.The nitride semiconductor according to claim 1, wherein the nitridesemiconductor has an RMS value of 5.0 nm or less obtained by a surfaceroughness measurement performed by an AFM.
 8. The nitride semiconductoraccording to claim 1, wherein the at least one group 13 element is Ga.9. The nitride semiconductor according to claim 1, wherein the nitridesemiconductor contains either one of, or both of, Si and Ge as donorimpurities.
 10. The nitride semiconductor according to claim 1, whereinthe specific resistance is 0.2×10⁻³ Ω·cm or higher.
 11. The nitridesemiconductor according to claim 1, wherein the specific resistance is0.15×10⁻³ Ω·cm or higher.
 12. The nitride semiconductor according toclaim 1, wherein the specific resistance is 0.1×10⁻³ Ω·cm or higher. 13.The nitride semiconductor according to claim 1, wherein the nitridesemiconductor fulfills a numerical range enclosed by four points atwhich: (a) the electron concentration is 1×10²⁰ cm⁻³ and the specificresistance is 0.3×10⁻³ Ω·cm, (b) the electron concentration is 3×10²⁰cm⁻³ and the specific resistance is 0.3×10⁻³ Ω·cm, (c) the electronconcentration is 4×10²⁰ cm⁻³ and the specific resistance is 0.15×10⁻³Ω·cm, and (d) the electron concentration is 9×10²⁰ cm⁻³ and the specificresistance is 0.15×10⁻³ Ω·cm.
 14. A contact structure comprising thenitride semiconductor according to claim 1 for a conductive portion. 15.A contact structure comprising the nitride semiconductor according toclaim 1 for an electrode.
 16. A semiconductor device comprising thecontact structure according to claim
 14. 17. A semiconductor devicecomprising the contact structure according to claim 15.